Other types of hydrogen bonds

A number of other types of hydrogen-bond formation between pyrimidines and purines have been discussed by Donohue.m... 

Multiple recognition and catalysis in ATP hydrolysis with increased ATP/ADP selectivity has been achieved with a multifunctional anion receptor containing a macrocyclic polyamine as anion binding site, an acridine group as stacking site and a catalytic site for hydrolysis (structure 82) [4.27]. Phosphoryl transfer is accelerated by other types of hydrogen-bonding receptors [5.24a]. 

Abstract The different C-H- X interactions are analyzed in this chapter. They are compared with the other types of hydrogen bonds, especially O-H- O ones. Theoretical tools are presented, among them the atoms in molecules theory (AIM). The H-bond motifs existing in crystals are presented. The evidences that C-H- -X interactions may be often classified as H-bonds are discussed. Etter s graph set analysis is also used to describe the structure of complex aggregates. 

A number of other types of hydrogen-bonded interactions betw een purines and pyrimidines have been discussed recently by Donohue (14). In the absence of evidence indicating that these structures occur in nature and of thoroughly reliable information about the dimensions of the molecules, we have not thought it w orth while to carry out a detailed metrical study of these structures. 

The proposed mechanism presents the nucleophile undergoing a formal Bronsted base interachon with the guanidine catalyst, thus activating the hydro-cyanate for addition to the stabilized electrophile. In comparison to bifunctional catalysts, the guanidines are basic enough to activate and stabiUze both nucleophile and electrophile without assistance from other types of hydrogen-bond interachons. 

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

This review will concentrate on metal-free Lewis acids, which incorporate a Lewis acidic cation or a hypervalent center. Lewis acids are considered to be species with a vacant orbital [6,7]. Nevertheless, there are two successful classes of organocatalysts, which may be referred to as Lewis acids and are presented in other chapter. The first type is the proton of a Brpnsted acid catalyst, which is the simplest Lewis acid. The enantioselectivities obtained are due to the formation of a chiral ion pair. The other type are hydrogen bond activating organocatalysts, which can be considered to be Lewis acids or pseudo-Lewis acids. 

Chloride ions have been determined by other studies to occupy the bridging sites between neighboring ligands shown in 40 and 41 in DMSO solution (106). This type of hydrogen bonding has been postulated to stabilize the A(d65) configuration in the Cf and Br crystals, but is absent in the 1 crystal (105). 

Another type of hydrogen bonding of oxetane is clathrate formation with water. Clathrate compositions of 1 6.5 and 1 17 moles of water have been observed <72CR(C)(274)1108). Oxetane has been found to be more effective than other cyclic ethers in solvating uranyl hexafluoroacetonylacetate (81IC1415). 

---